How Does Fluorouracil Work? Mechanism of Action Explained in Plain English

Fluorouracil (5-FU) has been treating cancer since 1962, but how it actually works inside your body is a fascinating story of molecular deception. Here's a plain-English breakdown of the mechanism of action of fluorouracil — why it works, why it has the side effects it does, and why a genetic enzyme deficiency can make it deadly.

The Big Picture: Fluorouracil Is a Trojan Horse

Cancer cells divide rapidly and constantly. To do that, they need to copy their DNA and build new cellular machinery. Fluorouracil works by disguising itself as a natural building block of DNA and RNA — and then breaking the machinery cancer cells use to replicate.

Fluorouracil belongs to a class of drugs called pyrimidine analogs. Pyrimidines are the natural building blocks of DNA — specifically, the bases uracil, cytosine, and thymine. Fluorouracil looks almost identical to uracil, except it has a fluorine atom attached. This tiny difference is enough to disrupt cells, but not enough that they can tell the difference and reject it.

Step 1: Fluorouracil Enters Cells and Gets Activated

Once fluorouracil enters a cell, enzymes convert it into two active metabolites that cause damage in different ways:

FdUMP (fluorodeoxyuridine monophosphate): This is the form that attacks DNA synthesis.

FUTP (fluorouridine triphosphate): This form gets incorporated into RNA, disrupting protein synthesis.

Step 2: FdUMP Blocks Thymidylate Synthase (The DNA-Building Enzyme)

To replicate DNA, cells need an enzyme called thymidylate synthase (TS). This enzyme converts a compound called dUMP into dTMP (thymidine monophosphate), which is a necessary building block of DNA.

FdUMP binds irreversibly to thymidylate synthase and locks it in an inactive state. The enzyme is essentially jammed. Without working thymidylate synthase, cells cannot make the thymidine they need to copy their DNA. Cell division stops, and the cell eventually dies.

This is why leucovorin (a form of folate) is often given alongside fluorouracil — leucovorin stabilizes the FdUMP-TS bond, making the inhibition stronger and more prolonged.

Step 3: FUTP Disrupts RNA and Protein Synthesis

At the same time, FUTP — another metabolite of fluorouracil — gets mistakenly incorporated into RNA molecules in place of normal uridine. RNA is the cell's messenger system, used to translate genetic instructions into proteins. When FUTP is embedded in RNA, the messages are corrupted. Cells can't make the proteins they need, further disrupting their ability to function and divide.

Why Does Fluorouracil Affect Normal Cells Too?

The attack on thymidylate synthase and RNA affects any rapidly dividing cell — not just cancer cells. This is why fluorouracil causes side effects primarily in tissues that normally regenerate quickly:

The lining of the mouth and gut (causing mouth sores and diarrhea).

Bone marrow (causing low blood cell counts).

Skin and hair follicles (causing skin changes and hair thinning).

The DPD Enzyme: Fluorouracil's Safety Valve

Fluorouracil is highly toxic in large amounts. Normally, the enzyme dihydropyrimidine dehydrogenase (DPD) breaks down more than 80% of fluorouracil into inactive metabolites. This enzymatic breakdown acts as a safety valve — controlling how much active drug is in your system.

In patients with a deficiency of DPD (caused by variants in the DPYD gene), this safety valve is broken. Fluorouracil is not broken down normally, so it accumulates to toxic levels. The result can be severe and potentially fatal side effects — severe mucositis, bone marrow failure, and neurological complications — even with a standard dose.

In February 2026, the FDA updated labeling for fluorouracil to more strongly recommend DPYD genetic testing before treatment begins. This is a critical safety step.

How Does Topical Fluorouracil Work on Skin?

When fluorouracil cream is applied to the skin, it works by the same mechanism locally — targeting rapidly dividing abnormal cells in actinic keratosis or superficial basal cell carcinoma. Precancerous and cancerous skin cells divide faster than surrounding healthy skin, making them more susceptible to fluorouracil's DNA-blocking effects. The skin reaction — redness, peeling, and crusting — is actually evidence that the drug is destroying abnormal cells.

For a complete guide to what fluorouracil treats, see What Is Fluorouracil? For details on side effects and what to watch for, see Fluorouracil Side Effects.